Cancer Stem Cell Research Leads to Clinical Trials

Dennis Slamon and Zev Wainberg from the UCLA Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research have been awarded a Disease Team Therapy Development award to begin clinical trials in human patients early in 2014.

In this clinical trial, Slamon and Wainberg will test a new drug that targets cancer stem cells. This drug was developed by research and development over the last decade on the cancer stem cell hypothesis. The cancer stem cell hypothesis predicts that proliferating stem cells are the main drivers of tumor growth and are also resistant to standard cancer treatments.  This new drug, CFI-400945, has prevented cancer growth in an extensive series of laboratory animal tests.

An important extension of the cancer stem cell hypothesis is that cancer stem cells inhabit a particular niche that prevents anticancer drugs from reaching them. Alternatively, tumors become resistant to cancer drugs by a process called “cell fate decision,” in which some cancer stem cells are killed by chemotherapy, but other cells replace them and repopulate the tumor. This tumor repopulation is the main reason for cancer recurrence.

The new anticancer drug to be tested in this clinical trial targets the “polo-like kinase 4.” Inhibition of this enzyme effectively blocks cell fate decisions that cause cancer stem cell renewal and tumor cell growth. Thus inhibition of this enzyme effectively stops tumor growth.

This clinical trial will test this novel chemotherapeutic agent in patients to establish the safety of the drug. After these initial safety tests, the trial will quickly proceed to further clinical tests. “We are excited to continue to test this drug in humans for the first time,” said Wainberg. Slamon, Wainberg and others will also look for biological markers to determine how well their drug is working in each patient.

The US Food and Drug Administration approved the Investigational New Drug (IND) for this drug trial. Also, Health Canada, the Canadian government’s therapeutic regulatory agency, also approved this trial. These approvals are part of an international effort to bring leading-edge stem cell science to patients.

A Stem Cell-Based Therapy for Colon Cancer

Colorectal cancer is the third leading cause of death in the Western World. Like many other types of cancer, colorectal cancer spreads and is propagated by cancer stem cells. Therefore, understanding how to inhibit the growth of cancer stem cells provides a key to treating the cancer itself.

By inactivating a gene that drives stem cell renewal in cancer stem cells, scientists and surgeons at the Princess Margaret Cancer Centre in Toronto, Canada, have discovered a promising new approach to treating colorectal cancer.

John Dick, a senior scientist at the Princess Margaret Cancer Centre, said, “This is the first step toward clinically applying the principles of cancer stem cell biology to control cancer growth and advance the development of durable cures.”

In preclinical experiments with laboratory rodents, Dick and his team identified a gene called BMI-1 as a pivotal regulator of colon cancer stem cell proliferation. With this knowledge in hand, Dick’s laboratory dedicated many hours to finding small molecules that disarm BMI-1. Then Dick and his co-workers replicated human colorectal cancer in mice, and used their BMI-1-inhibiting small molecules to treat these cancer-stricken mice.

According to lead author of this work, Antonija Kreso: “Inhibiting a recognized regulator of self-renewal is an effective approach to control tumor growth, providing strong evidence for the clinical relevance of self-renewal as a biological process for therapeutic targeting.”

Dr. Dick explained: “When we blocked the BMI-1 pathway, the stem cells were unable to self-renew, which resulted in long-term and irreversible impairment of tumor growth. In other words, the cancer was permanently shut down.”

The clinical potential of this approach is significant, since it provides a viable treatment that specifically targets colon cancer. About 65% of all colorectal cancers have an activated BMI-1 pathway. Since physicians now have techniques for identifying the presence of BMI-1 and the tools to inhibit it, this strategy could translate into a clinical treatment that might radically transform the treatment of aggressive, advanced colorectal cancers. Such a treatment would be specific, personal, and specific. May the phase 1 trials begin soon!!!

Breast Cancer Clinical Trial Targets Cancer Stem Cells

Even though my previous posts about cancer stem cells have generated very little interest, understanding cancer as a stem cell-based disease has profound implications for how we treat cancer. If the vast majority of the cells in a tumor are slow-growing and not dangerous but only a small minority of the cells are rapidly growing and providing the growth the most of the tumor, then treatments that shave off large numbers of cells might shrink the tumor, but not solve the problem, because the cancer stem cells that are supplying the tumor are still there. However, if the treatment attacks the cancer stem cells specifically, then the tumor’s cell supply is cut off and the tumor will wither and die.

In the case of breast cancer, the tumors return after treatment and spread to other parts of the body because radiation and current chemotherapy treatments do not kill the cancer stem cells.

This premise constitutes the foundation of a clinical trial operating from the University of Michigan Comprehensive Cancer Center and two other sites. This clinical trial will examine a drug that specifically attacks breast cancer stem cells. The drug, reparixin, will be used in combination with standard chemotherapy.

Dr. Anne Schott, an associate professor of internal medicine at the University of Michigan and principal investigator of this clinical trial, said: “This is one of only a few trials testing stem cell directed therapies in combination with chemotherapy in breast cancer. Combining chemotherapy in breast cancer has the potential to lengthen remission for women with advanced breast cancer.”

Cancer stem cells are the small number of cells in a tumor that fuel its growth and are responsible for metastasis of the tumor. This phase 1b study will test reparixin, which is given orally, with a drug called paclitaxel in women who have HER2-negative metastatic breast cancer. This study is primarily designed to test how well patients tolerate this particular drug combination. However, researchers will also examine how well reparixin appears to affect various cancer stem cells indicators and signs of inflammation. The study will also examine how well this drug combination controls the cancer and affects patient survival.

This clinical trial emerged from laboratory work at the University of Michigan that showed that breast cancer stem cells expressed a receptor on their cell surfaces called CXCR1. CXCR1 triggers the growth of cancer stem cells in response to inflammation and tissue damage. Adding reparixin to cultured cancer stem cells killed them and reparixin works by blocking CXCR1.

Mice treated with reparixin or the combination of reparixin and paclitaxel had significantly fewer (dramatically actually) cancer stem cells that those treated with paclitaxel alone. Also, riparixin-treated mice developed significantly fewer metastases that mice treated with chemotherapy alone (see Ginestier C,, et al., J Clin Invest. 2010, 120(2):485-97).

Isolating Mammary Gland Stem Cells

Female mammary glands are home to a remarkable population of stem cells that grow in culture as small balls of cells called “mammospheres.” Clayton and others were able to identify these stem cells in 2004 (Clayton, Titley, and Vivanco, Exp Cell Res 297 (2004): 444-60), and Max Wicha’s laboratory at the University of Michigan showed that a signaling molecule called Sonic Hedgehog and a Polycomb nuclear factor called Bmi-1 are necessary for the self-renewal of normal and cancerous mammary gland stem cells (Lui, et al., Cancer Res June 15, 2006 66; 606). The biggest problem with mammary gland stem cells is isolating them from the rest of the mammary tissue.

Mammary gland stem cells or MaSCs are very important for mammary gland development and during the induction of breast cancer. Getting cultures of MsSCs is really tough because the MaSCs share cell surface markers with normal cells and they are also quite few in number.

Gregory Hannon and his co-workers at Cold Spring Harbor Laboratory used a mouse model to identify a novel cell surface protein specific to MaSCs. By exploiting this unusual marker, Hannon and his team were able to isolate MaSCs from mouse mammary glands of rather high purity.

Camila Do Santos, the paper’s first author, said that “We are describing a marker called Cd1d.” Cd1d is also found on the surfaces of cells of the immune system, but is specific to MaSCs in mammary tissue. Additionally, MaSCs divide slower than the surrounding cells. Do Santos and her colleagues used this feature to visually isolate MaSCs from cultured mammary cells.

They used a mouse strain that expresses a green glowing protein in its cells and then made primary mammary cultures from these green glowing mice. After shutting of the expression of the green glowing protein with doxycycline, the cultured cells divided, and diluted the quantity of green glow protein in the cells. This caused them to glow less intensely. However, the slow-growing MaSCs divided much more slowly and glowed much more intensely. Selecting out the most intensely glowing cells allowed Dos Santos and her colleagues to enrich the culture for MaSCs.

“The beauty of this is that by stopping GFP expression, you can directly measure the number of cell divisions that have happened since the GFP was turned off,” said Dos Santos. She continued: “The cells that divide the least will carry GFP the longest and are the ones we characterized.”

Using this strategy, Dos Santos and others selected stem cells from the mammary glands in order to examine their gene expression signature. They also confirmed that by exploiting Cd1d expression in the MaSCS, in combination with other techniques, they could enhance the purity of the cultures several fold.

Hannon added, “With this advancement, we are now able to profile normal and cancer stem cells at a very high degree of purity, and perhaps point out which genes should be investigated as the next breast cancer drug targets.”

Will we be able to use these cell for therapeutic purposes some day?  Possibly, but at this time, more must be known about them and MaSCs must be better characterized.

Keeping Stem Cells Stem Cells

Chengcheng Zhang is an assistant professor in the UT Southwestern Medical Center departments of physiology and developmental biology in Dallas, Texas. His lab has identified a receptor on the surface of cancer stem cells that, when activated, prevents them from differentiating.

Zhang explains his work this way: “Cancer cells grow rapidly in part because they fail to differentiate into mature cells. Drugs that induce differentiation can be used to treat cancers.” In his however has identified a new target for cancer: “Our research has identified a protein receptor on cancer cells that induces differentiation, and knowing the identity of this protein should facilitate the development of new drugs to treat cancers.”

The receptor to which Zhang is referring is a member of a family of proteins known as the “leukocyte immunoglobulin-like receptors.” These LIRs, as they are called, have bits located outside the cell and help regulate cells of the immune system. The LIR that Zhang’s lab found is called the subfamily B member 2 or LILRB2. LILRB2 is found on the surface of immune cells where it binds to molecules on the surface of cells that process antigens (foreign substances in the body) and prevents the initiation of an immune response. LILRB2 also has a newly-described role in stem cell biology.

Zhang again: “The receptor we identified turned out to be a protein called a classical immune inhibitory receptor, which is known to maintain stemness of normal adult stem cells and to be important in the development of leukemia.”

What does Zhang mean by “stemness?” He is referring to the potential of a bone marrow stem cell that makes blood cells to develop into different kinds of cells and replenish red blood cells lost to wear and tear or injury. Once stem cells differentiate into adult cells, they cannot return to their original stem cell state. The body seems to only have a finite number of stem cells and, therefore, depleting them is unwise.

Before Zhang’s study, there was no indication that LILRB2 could bind to anything but surface proteins on antigen-presenting cells, but Zhang and his team has discovered a new function for LILRB2. LILRB2 can bind to members of a poorly understood group of proteins known as angiopoietic-like proteins that support stem cell growth. By binding angiopoietic-like proteins, LILRB2 sends a signal to the interior of the stem cell to not differentiate. This inhibition keeps cancer stem cells from differentiating. By not differentiating, the stem cells divide furiously and never differentiate and make progeny cells that also divide many times and do not differentiate. This is the main mechanism that drives the progression of leukemia.

Zhang said that this inhibition does not cause cancer stem cells to make new stem cells but does not preserve their potential to do so. Also, making inhibitors that prevent the interaction between angiopoietin-like proteins and LILRB2 can force cancer stem cells to differentiate. Thus these new findings may give us a target for fighting leukemia.